1. Field of the Invention
The present invention relates to a physiological recording electrode, and, more particularly, to an EEG (electroencephalography) recording electrode that can be used without the need for numerous steps in preparing the subject's skin and the electrode itself. The invention further relates to a surface feature or penetrator with a size and shape which that will not bend or break, which limits the depth of application, and/or anchors the electrode or other device during normal application; and a packaging system comprising a well and electrolytic fluid for maintaining a coating of said electrolytic fluid on the surface feature or penetrator.
2. Technology Review
Electrodes for measuring biopotential are used extensively in modern clinical and biomedical applications. These applications encompass numerous physiological tests including electrocardiography (ECG), electroencephalography (EEG), electrical impedance tomography (EIT), electromyography (EMG) and electro-oculography (EOG). The electrodes for these types of physiological tests function as a transducer by transforming the electric potentials or biopotentials within the body into an electric voltage that can be measured by conventional measurement and recording devices.
In general, most commercial EEG electrodes for these applications today are placed on the surface of the skin, which is a layered structure consisting of the epidermis and the dermis. The dermis contains the vascular and nervous components. Further it is the part of the skin where pain has its origins. The epidermis, however, contains no vascular or nervous components and is made up of several layers, including the Stratum basale or stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum, and the stratum corneum.
The stratum corneum, the outermost layer of the skin, is the primary source of high electrical impedance and, thus, this layer dramatically influences the biopotential measurements. The stratum corneum is very thin and uniform in most regions of the body surface ranging from 13-15 μm with a maximum of about 20 μm. If the high impedance results from the stratum corneum can be reduced, a more stable electrode will result. Therefore with existing physiological electrodes the skin must be prepared prior to application when lower impedance is required.
The most common electrode preparation methods to avoid the high impedance effects of the stratum corneum are: 1) shaving the hair from the skin; and either 2a) abrading the stratum corneum or 2b) using an electrolytic gel. Electrodes requiring the use of an electrolytic gel or fluid are often referred to as “wet” electrodes. Hair is shaved from the skin to improve the contact between the electrodes and the skin surface. The goal of the abrasion of the stratum corneum is to reduce the thickness of (or remove) the stratum corneum (and therefore its electrically insulating characteristics).
Drawbacks of abrading the skin are that the abraded area regenerates dead cells fairly quickly (resulting in a limited time period for using the electrode), and if the abrasion is too deep the person can experience pain. Additionally, electrolytic gels or fluids may be applied to abraded surface to enhance the contact. Alternatively, electrolytic gels or fluids can be applied to the surface of the skin directly. The electrolytic gel having a high concentration of conductive ions diffuses into the stratum corneum and improves its conductivity. Drawbacks observed with the use of electrolytic gels or fluids involve the change of conductivity with time as the gels dry, discomfort (an itching sensation) at the patient's skin as a result of the gels drying, and the possibility of a rash due to an allergic reaction to the electrolytic gels.
Further drawbacks of “wet” electrodes include skin preparation and stabilization of the electrode with respect to the skin surface. This is because movement of the electrode on the surface of the skin causes the thickness of the electrolytic layer (formed by the electrolytic gels or fluids) to change resulting in false variation in the measured biopotential. Some electrode designs have an adhesive backing to reduce the movement of the electrode on the skin surface; however, this feature does not eliminate completely the movement of the electrode with respect to the subject's skin. Another drawback is the length of time required to prepare the skin and apply the electrolytic gels or fluids prior to measurement of the biopotentials.
More recently, dry electrodes have been developed which eliminate many of these limitations by foregoing the need for electrolytic fluids, gels, or colloids. For example, in Schmidt (U.S. Pat. No. 6,782,283) a dry electrode containing a surface feature or penetrator is used to penetrate the stratum corneum of the skin and conduct a signal without the aid of electrolytic fluid. The design of Schmidt is such that the electrode's surface feature or penetrator(s) pierce, break, or create entry through the high impedance layers of the subject's skin, and thus come in contact with the more electrically conductive layers which facilitates the transmission of biopotential signals.
The downside to Schmidt is that it teaches against the use of electrolytic fluid, gel or colloid at all. However, use of a dry electrode with such electrolytic fluid, gel, or colloid unexpectedly enhances the conductivity of biopotential signals being collected from a subject, particularly with weaker biopotential signals such as EEG signals.
In view of the foregoing inherent disadvantages with presently available wet and dry electrodes, it has become desirable to develop an electrode that does not require skin preparation or the use of electrolytic gels and overcomes the inherent disadvantages of presently available dry electrodes.